The X1900XT/XTX is a X1800XT with its shader ALU's tripled. We wont see this 300% theroritical performance enhancment for a good while in games though, as most games are a little bit away from being that shader heavy, but shaders are the future.
As for the 7800, the GTX in 256MB has a good value, but its image quality is VERY lacky compared to ATi, same goes for the 512MB version which just happens to be impossible to get for a normal consumer and even if you find it instock you'll be paying around $800 for a card out performaned by the X1900 serious which is cheaper.
Nvidia will be launching a 7900 soon, we'll see how it performans, but that means jack **** if they dont improve their image quality soon. Nvidia has bad texture filtering and also has bad cases of texture poping and shimmering on textures. So we'll see how that turns out.
Overall a good launch for ATi.
Also, those who are interested in more a technical article and in depth explanation (with assumed knowledge of how a GPU architecture works) might enjoy the Beyond3D article here:
http://beyond3d.com/reviews/ati/r580/
Also, here's the HardOCP article, usually good game tests etc, but their summaries show their complete lack of knowledge beyond reading numbers off a FPS counter:
http://hardocp.com/article.html?art=OTUzLDE=